Semi-crystalline polymers are useful as engineering thermoplastics because they possess advantageous chemical, physical and electrical properties. Semi-crystalline thermoplastic polymers, for instance, can be readily processed by thermal means and formed into numerous and different shapes. For instance, thermoplastic polymers can be formed into various articles through one of many different molding processes such as extrusion, rotational molding, blow molding, and injection molding.
One particular type of injection molding that is used to produce plastic articles is typically referred to in the art as fluid-assisted injection molding. In fluid-assisted injection molding, a molten polymer composition is injected into a mold in conjunction with a fluid, such as a gas. The fluid is injected into the mold under pressure placing a force on the molten polymer composition. In this manner, the fluid not only forces the polymeric composition into the extremities of the mold, but also creates an internal hollow cavity or void space in the resulting article. During the process, a single fluid can be introduced into the mold or multiple fluids. For instance, in one particular embodiment, a gas is injected into the mold followed by a liquid.
The fluid maintains pressure against the polymeric composition until the polymeric composition substantially cools. Once the article is cooled, the fluid pressure is reduced and the article is removed from the mold.
During fluid-assisted injection molding, the fluid provides numerous benefits. For instance, by forming a hollow cavity or void on the interior of the article, less plastic is used to produce the article and the resulting article is lighter. The fluid can also be used to speed up the cycle time or the time it takes to produce the article. As described above, the fluid also forces the polymer composition into the different parts of the mold and potentially improves the surface characteristics of the resulting article.
The present disclosure is directed to further improvements in fluid-assisted injection molding processes. In particular, in the past, although fluid-assisted injection molding can produce products having relatively favorable surface characteristics, pitting on the surface still remains a problem, especially when producing fiber reinforced articles. The present disclosure is directed particularly to the production of fluid-assisted injection molding articles that have reduced surface imperfections, such as pits or glass fiber on the surface.